During the normal sliding or rolling of one metal part against another, tiny metallic wear particles are produced in various sizes and shapes. Where the metal parts are lubricated by oil or some other hydraulic fluid the particles produced due to wear are either suspended in the lubricant, forming a colloid that can be tolerated until the concentration of wear particles become too large, or transported through the system until captured in a filter. Frequently, particularly in the case of old machines such as engines, transmissions, and gear boxes, larger particles, termed "chips", are produced as the internal parts wear out. When these chips are present they indicate equipment damage and must be removed since they can cause additional damage to the equipment.
When it is necessary to determine the presence of wear particles in machine lubricants, wear particle detectors are employed. For over a decade such electric chip detectors have been installed in helicopter transmissions and engines, as well as in propulsion and drive systems of many military fixed wing aircraft. Since the metallic parts are composed of steel or alloys of steel, the detectors usually contain a magnet that attracts the ferrous debris shed into the lube system by failing components. In detail, detectors usually have a pair of permanently magnetized electrodes that are insulated from each other and positioned to form a magnetic field in the small gap therebetween. One electrode is usually electrically grounded and the other electrode is connected to an electrical warning or fuzz burn-off circuit, one of which is a capacitance circuit described in U.S. Pat. No. 4,070,660. Such circuits operate by passing a small current through the chip or metallic debris that collect and eventually bridges the gap between the two electrodes.
One of the underlying problems of monitoring wear particles is that one cannot see the wearing surfaces, and the existing detectors do not distinguish between the larger harmful particles or chips and the accumulations of harmless minute particles such metal dusts called fuzz. Consequently, detectors can be triggered by an accumulation of metallic dust, causing a false chip detection warning. When such a warning signal is received the equipment must be shut-down and inspected to determine the cause of the signal. In the case of a helicopter gear box the false chip indication causes a mission to be aborted by the immediately required landing. Some improved systems are adapted to burnoff a certain class of metallic debris (e.g., long hair-like strands) to reduce the occurence of false warnings. However, such systems accommodate only limited types of debris and, as set forth in U.S. Pat. No. 4,070,660 destroy fuzz by the discharging of a capacitor through the fuzz in order to melt it. If the capacitor does not melt the debris, thus opening the bridge across the gap, then the capacitor cannot recharge and a signal is activated indicating the presence of a chip. This detector has the disadvantage that the debris must have a suitable geometry to be able to suddenly apply a low impedance across the chip detector gap. Long hair-like strands are examples because they can be captured by the magnetic field and suddenly bridge the two electrodes. Then a full capacitor charge can be discharged through the strand to melt it away. Agglomerations of small particles tend to slowly reduce the gap resistance and weaken the capacitor charge; often this class of debris does not burnoff and results in false warnings. In addition, a separate burnoff circuit is incorporated in each wear particle detector of U.S. Pat. No. 4,070,660. This is undesirable where as many as seven detectors may be located on a single helicopter. It would be more desirable to have a single circuit for all seven detectors. It would be even more advantageous to have a system sensitive to the size of the particle captured by the detector, and that indicates to the crew when the detector is becoming clogged with small debris. A wear particle detecting system is provided herein which accomplishes all of these advantages.